Projection-type charged particle optical system and imaging mass spectrometry apparatus

ABSTRACT

Provided is a projection-type charged particle optical system in which a projection magnification can be changed while a decrease in the accuracy in measuring a mass-to-charge ratio is being suppressed. A projection-type charged particle optical system according to the present invention includes a first electrode disposed so as to face a sample and having an opening formed therein for allowing a charged particle to pass, a second electrode disposed on a side of the first electrode opposite to where the sample is disposed and having an opening formed therein for allowing the charged particle to pass, and a flight-tube electrode disposed such that the charged particle that has been emitted from the sample and has passed through the second electrode enters the flight-tube electrode and being configured to form a substantially equipotential space thereinside. A principal plane is formed at at least two positions in a travel path of the charged particle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/901,672, filed Dec. 28, 2015, which is a national stage ofApplication No. PCT/JP2015/056445 filed Feb. 26, 2015 and which claimsthe benefit of Japanese Patent Application No. 2014-040967, filed Mar.3, 2014 and No. 2015-033378, filed Feb. 23, 2015, all of which arehereby incorporated by reference herein in their entirety. U.S. patentapplication Ser. No. 14/901,672 contains the certified copies ofJapanese Patent Application No. 2014-040967 and No. 2015-033378.

TECHNICAL FIELD

The present invention relates to time-of-flight mass spectrometryapparatuses configured to carry out mass spectrometry by ionizing atleast a portion of a sample and measuring the time of flight of thegenerated ions.

BACKGROUND ART

Imaging mass spectrometry is receiving broad attention as a techniquefor detecting the distribution of substances within a sample, in thefield of pathological studies, drug development, and so on. Typically,in mass spectrometry, a sample is irradiated with a laser beam, an ionbeam, an electron beam, or the like so as to be ionized, and theresulting ions are separated in accordance with their mass-to-chargeratios. Thus, a spectrum of detected intensities as a function of themass-to-charge ratio is obtained. In imaging mass spectrometry, asurface of an object to be measured (sample) is subjected to massspectrometry two-dimensionally, and a two-dimensional distribution ofdetected intensities of the substances corresponding to the respectivemass-to-charge ratios is obtained. Thus, information on the distributionof the substances at the surface of the sample is obtained. Imaging massspectrometry enables biomolecules, such as protein, and drug moleculesto be identified, and also makes it possible to measure the spatialdistribution of such biomolecules and drug molecules at high spatialresolution.

A charged particle beam, such as a laser beam and an ion beam, is usedin order to ionize a sample, and such a beam is generally referred to asa primary beam. In addition, an ion generated when an ion beam is usedas a primary beam (primary ion beam) is referred to as a secondary ion,and a technique for detecting such secondary ions is known assecondary-ion mass spectrometry (SIMS). Furthermore, matrix-assistedlaser desorption/ionization (MALDI) is a known example in which a laserbeam is used as a primary beam, and a sample that has been crystallizedby being mixed into a matrix is irradiated with a pulsed laser beam soas to be ionized.

A time-of-flight technique, which is suitable for detecting moleculeshaving large mass, such as protein, is often employed for detecting anionized sample by separating the ionized sample in accordance with themass-to-charge ratio. In a time-of-flight mass spectrometry apparatus,ions are generated in pulses at a surface of a sample, and acceleratedthrough an electric field; or ions are accelerated in pulses through anelectric field in vacuum. Ions travel at different flight speeds inaccordance with their mass-to-charge ratios, and thus the mass-to-chargeratio of a given ion can be determined by measuring the time (i.e., timeof flight) it takes for the given ion to travel a predetermined distanceto reach a detector after being emitted from the sample.

In addition, imaging mass spectrometry includes scanning imaging massspectrometry and projection-type imaging mass spectrometry.

In scanning imaging mass spectrometry, small regions (the size isdependent on the beam diameter of a primary beam) on a sample aresuccessively subjected to mass spectrometry, and the distribution ofsubstances is reconstructed from the result of the mass spectrometry andthe positional information of the small regions. Thus, the spatialresolution is determined by the beam diameter of the primary beam andthe positional precision of the scanning primary beam.

In projection-type imaging mass spectrometry, a sample is irradiatedwith a primary beam so as to be ionized, and a position- andtime-sensitive detector detects the time at which a generated ion hasreached the detector and the position on the detection surface of thedetector which the ion has reached. Furthermore, a projection-typecharged particle optical system configured to form an image of the ionon the detector is provided, and thus the mass of the detected ion andthe position of the ion on the surface of the sample are detectedsimultaneously. Accordingly, the spatial distribution of substancescontained in the sample can be obtained. The projection-type chargedparticle optical system is formed by an electrostatic lens, a magneticlens, or the like.

An electrostatic lens is often used as a projection-type chargedparticle optical system (PTL1). An electrostatic lens causes ions toconverge through an electric field and forms an image of the ions on adetector. The spatial resolution of a projection-type charged particleoptical system is determined by the accuracy in determining thepositions on the detection surface where the ions have reached(positional resolution), the magnification or the aberration of thecharged particle optical system, and so forth.

In the field of pathological studies or drug development, a minutestructure in the order of microns needs to be observed when a cell or amicrostructure is to be observed. In the meantime, a broader range inthe order of millimeters or centimeters needs to be observed when abiological tissue is to be observed. Therefore, it is effective toswitch the magnification in such a manner that, in the case of theformer, the measurement is carried out in a condition in which thespatial resolution is high (high magnification), and in the case of thelatter, the measurement is carried out in a condition in which thespatial resolution is low but the measurement range is broad (lowmagnification).

CITATION LIST Patent Literature

PTL1 Japanese Patent Laid-Open No. 2010-251174

Technical Problem

In a projection-type imaging mass spectrometry apparatus, in order tocarry out measurements with high magnification and low magnification, aprobable technique to employ is to vary the magnification of aprojection-type charged particle optical system. A projection-typecharged particle optical system is formed by an extraction electrode, anelectrostatic lens, and the like, and electric fields generated amongthe lenses function as a convex lens as a whole. Such a convex lensrefracts the trajectory of an ion through one or more electric field(s),and an imaginary refractive plane obtained when such a refractivefunction of the convex lens as a whole is considered as a singleinstance of refraction is referred to as a principal plane in thepresent specification. In addition, voltages to be applied to theextraction electrode, the electrostatic lens, and so on are designedsuch that an image is formed appropriately by the charged particleoptical system as a whole.

When the voltage being applied to the extraction electrode or theelectrostatic lens is changed, an electric field present between theextraction electrode and the electrostatic lens changes accordingly, andthus the position of the principal plane in the charged particle opticalsystem can be changed along the axis of the charged particle opticalsystem. For example, when the position of the principal plane is broughtcloser to the object to be measured (sample), the magnificationincreases; whereas, the position of the principal plane is broughtcloser to the detector, the magnification decreases. Such an adjustmentcorresponds to varying a focal distance of the projection-type chargedparticle optical system. Therefore, adjusting the voltages asappropriate makes it possible to form an image of ions on the detectorand to adjust the magnification in accordance with a change in theposition of the principal plane.

In the meantime, in a time-of-flight mass spectrometry apparatus, an ionthat has been accelerated through an appropriate electric field formedby an extraction electrode and the like enters a flight-tube electrodethat is set to an appropriate potential. The ion then moves uniformly anappropriate distance, and is detected by a detector. Kinetic energywhich the ion gains through acceleration is constant regardless of themass-to-charge ratio of the ion, if the electric field stays constant.At least a portion of the internal space of the flight-tube electrode isa space in which the potential within the space is constant(equipotential space). In the equipotential space, an ion movesuniformly both in its flight direction and in a direction orthogonal tothe flight direction.

A reason for securing an equipotential space inside the flight-tubeelectrode is as follows. A time-of-flight mass spectrometry apparatusutilizes a principle that the flight speeds of ions having the samekinetic energy differ in accordance with their mass-to-charge ratios,and thus it is desirable that the kinetic energy of an ion travelinginside the flight-tube electrode do not change. However, when apotential inside the flight-tube electrode changes, the kinetic energyof the ion also changes, and thus the relationship between themass-to-charge ratio of the ion and its time of flight changes as well.Consequently, it becomes difficult to obtain an accurate mass-to-chargeratio, and thus it is desirable that the interior of the flight-tubeelectrode can be treated as a substantially equipotential space in whichthe potential is substantially constant. In order to set the potentialinside the flight-tube electrode to be constant, the potential of theflight-tube electrode itself may be held constant. However, it isnecessary to take an influence of a potential outside the flight-tubeelectrode into consideration as well.

Here, a way to adjust the position of a principal plane PP for changingthe magnification of a projection optical system in a conventionaltime-of-flight projection-type imaging mass spectrometry apparatus (FIG.2A) will be discussed. In a high-magnification condition (FIGS. 2B and2D), a strong electric field is present between an extraction electrode41 and a first projection electrode 42, and a principal plane PPH isgenerated on a nearby side of the first projection electrode 42 thatopposes a flight-tube electrode 45. Meanwhile, in a low-magnificationcondition (FIG. 2C), a principal plane is moved toward a detector byadjusting respective voltages in such a manner that an electric fieldpresent between the first projection electrode 42 and the flight-tubeelectrode 45 becomes stronger than an electric field present between theextraction electrode 41 and the first projection electrode 42. In thiscase, spacing between equipotential lines C inside the flight-tubeelectrode 45 in the low-magnification condition (FIG. 2E) is smallerthan that in the high-magnification condition (FIG. 2D). An electricfield that leaks into the flight-tube electrode 45 increases asillustrated in FIG. 2E, as compared with the case illustrated in FIG.2D, and thus the potential inside the flight-tube electrode 45 changes,causing the flight speed of an ion to change. Thus, due to the reasondescribed above, changing the magnification results in a problem in thatthe accuracy in measuring the mass-to-charge ratio is deteriorated.

As described thus far, the conventional time-of-flight projection-typeimaging mass spectrometry apparatus faces a challenge in changing theprojection magnification of the projection-type charged particle opticalsystem while suppressing a decrease in the accuracy in measuring themass-to-charge ratio.

SUMMARY OF INVENTION

The present invention provides a projection-type imaging massspectrometry apparatus that is capable of measuring a mass-to-chargeratio with high accuracy even when a projection magnification ischanged.

A projection-type charged particle optical system according to thepresent invention includes a first electrode disposed so as to face asample and having an opening formed therein for allowing a chargedparticle to pass therethrough, a second electrode disposed on a side ofthe first electrode that is opposite to where the sample is disposed andhaving an opening formed therein for allowing the charged particle topass therethrough, and a flight-tube electrode disposed such that thecharged particle that has been emitted from the sample and has passedthrough the first and second electrodes enters the flight-tube electrodeand being configured to form a substantially equipotential spacethereinside. A principal plane is formed at at least two positions in atravel path of the charged particle.

With the projection-type charged particle optical system according tothe present invention, the mass-to-charge ratio can be measured withhigh accuracy even when the projection magnification is changed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates a projection-type imaging mass spectrometryapparatus according to the present invention.

FIG. 1B illustrates a projection-type charged particle optical systemaccording to a first exemplary embodiment.

FIG. 1C illustrates potentials of respective electrodes in ahigh-magnification mode.

FIG. 1D illustrates potentials of respective electrodes in alow-magnification mode.

FIG. 1E illustrates a projection-type charged particle optical systemthat includes an aperture-equipped flight-tube electrode.

FIG. 2A illustrates a conventional projection-type imaging massspectrometry apparatus.

FIG. 2B illustrates a result of an ion-optical simulation in ahigh-magnification condition.

FIG. 2C illustrates a result of an ion-optical simulation in alow-magnification condition.

FIG. 2D illustrates a potential distribution in the high-magnificationcondition.

FIG. 2E illustrates a potential distribution in the low-magnificationcondition.

FIG. 3A illustrates a result of an ion-optical simulation in ahigh-magnification mode according to the first exemplary embodiment.

FIG. 3B illustrates the result of the ion-optical simulation in thehigh-magnification mode, and illustrates the vicinity of a sample.

FIG. 3C illustrates the position of a principal plane in thehigh-magnification mode.

FIG. 3D illustrates a potential distribution in the high-magnificationmode.

FIG. 4A illustrates a result of an ion-optical simulation in alow-magnification mode according to the first exemplary embodiment.

FIG. 4B illustrates the result of the ion-optical simulation in thelow-magnification mode, and illustrates the vicinity of a sample.

FIG. 4C illustrates a potential distribution in the low-magnificationmode.

FIG. 5A illustrates a result of an ion-optical simulation in ahigh-magnification mode according to a second exemplary embodiment.

FIG. 5B illustrates a result of an ion-optical simulation in alow-magnification mode.

FIG. 5C illustrates a result of an ion-optical simulation in amedium-magnification mode.

FIG. 5D illustrates a potential distribution in the high-magnificationmode.

FIG. 5E illustrates a potential distribution in the low-magnificationmode.

FIG. 5F illustrates a potential distribution in the medium-magnificationmode.

FIG. 6A is a schematic diagram of a conical electrode.

FIG. 6B is a schematic diagram of an aperture electrode.

FIG. 6C illustrates a cylindrical flight-tube electrode.

FIG. 6D illustrates a flight-tube electrode that is formed by aplurality of cylindrical electrode.

FIG. 6E illustrates another flight-tube electrode that is formed by aplurality of aperture electrodes.

FIG. 6F illustrates a projection-type imaging mass spectrometryapparatus according to a third exemplary embodiment.

FIG. 6G illustrates a projection-type charged particle optical systemaccording to a fourth exemplary embodiment.

FIG. 7A illustrates a result of an ion-optical simulation in ahigh-magnification mode according to the fourth exemplary embodiment.

FIG. 7B illustrates a result of an ion-optical simulation in alow-magnification mode.

FIG. 7C illustrates a potential distribution in the high-magnificationmode.

FIG. 7D illustrates a potential distribution in the low-magnificationmode.

DESCRIPTION OF EMBODIMENTS First Exemplary Embodiment

A projection-type imaging mass spectrometry apparatus according to anexemplary embodiment of the present invention is illustrated in FIG. 1A.

The projection-type imaging mass spectrometry apparatus according to thepresent exemplary embodiment includes an ion gun 1 that irradiates asample 2 with primary ions serving as a primary beam, a sample stage 3that supports the sample 2, a projection-type charged particle opticalsystem 4 disposed so as to face the sample 2, and a position- andtime-sensitive ion detector 5 that detects secondary ions. The ion gun1, the sample stage 3, the charged particle optical system 4, and theion detector 5 constitute a vacuum vessel 6. Although not illustrated,the projection-type imaging mass spectrometry apparatus further includesa vacuum pumping system and a signal processing system.

The charged particle optical system 4 and the ion detector 5 constitutea time-of-flight mass spectrometer. An image of charged particles isformed on a surface of the ion detector 5, and the time at which thecharged particles, which contain the secondary ions, have been detectedis recorded in the ion detector 5.

A cluster-ion gun that supplies cluster ions generated from varioustypes of gases is used as an example of the ion gun 1 according to thepresent exemplary embodiment. The present exemplary embodiment, however,is not limited thereto, and a liquid-metal ion source, a duoplasmatron,a surface-ionization ion source, or the like may instead be used.

In addition, the source for the primary beam may be a charged particlesource that irradiates the sample 2 so as to generate charged particlesfrom the surface of the sample 2, and the primary beam may be anelectromagnetic wave, such as a laser beam, or a charged particle beam,such as an ion beam. The charged particle source may be a pulsed chargedparticle source that generates charged particles from the surface of thesample 2 in pulses, and a pulsed laser-beam source or a pulsed ionsource may be used as such a pulsed charged particle source.

The ion gun 1 includes a nozzle 11, an ionization unit 12, a massselector 13, a chopper 14, and a primary-ion lens 15 (FIG. 1A).

A noble gas (e.g., Ar, Ne, He, and Kr), a molecular gas (e.g., CO₂, CO,N₂, O₂, NO₂, SF₆, Cl₂, and NH₄), an alcohol (e.g., ethanol, methanol,and isopropyl alcohol), or water is supplied to the nozzle 11 through agas-introduction pipe. An acid or a base may be mixed into water or analcohol. The pressure at which a gas is introduced is not particularlimited, and may be in a range from 0.001 atm to 100 atm. Preferably,the pressure may be in a range from 0.1 atm to 20 atm.

When a gas is injected into a vacuum through the nozzle 11, the suppliedgas, or a liquid, is accelerated to a supersonic speed. At that time,the gas is cooled through adiabatic expansion; and a gas that contains acluster, which is an aggregate of atoms or molecules, is generated. Atleast one of the cluster and the gas enters the ionization unit 12. Anelectron source, such as a hot filament, is disposed in the ionizationunit 12. An atom or a molecule that is contained in the cluster isionized by an electron emitted from the electron source; and a clusterion is thus generated.

Cluster ions and monomer ions in various sizes are generated in theionization unit 12; and such cluster ions and monomer ions areaccelerated as appropriate, and then enter the mass selector 13. Thus, acluster ion beam A having a desired size is generated. The mass selector13 may be a time-of-flight mass selector, a quadrupole mass selector, ora magnetic mass selector.

The cluster ion beam A is turned into a pulsed cluster ion beam A by thechopper 14. The pulsed cluster ion beam A can also be obtained by usinga nozzle through which a gas is injected in pulses or by using anionization unit that ionizes a cluster in pulses, in place of thechopper 14. The chopper 14 may be capable of operating with a pulseduration of several tens of nanoseconds or shorter at a higher pulserate.

The acceleration energy of the cluster ion beam A is in a range fromseveral [keV] to several tens of [keV]; however, the acceleration energymay exceed several tens of [keV], to improve the convergence of theprimary ion beam or the generation efficiency of the secondary ions.

In the meantime, it is preferable that the acceleration energy per atomor molecule of the cluster ion be no more than 8.3 [eV]. In such a case,the secondary ions can be generated in a condition in which dissociationof an object to be measured in the sample 2 is being suppressed, whichenables so-called soft ionization. Thus, the mass of macromolecules,such as protein, can be measured at high sensitivity. Similarly, sucheffects can be expected that dissociation of a C—H bond, a C—C bond, aC—O bond, and a C—N bond are suppressed if the acceleration energy ofthe cluster ion per atom or molecule is, respectively, 4.3 [eV], 3.6[eV], 3.4 [eV], and 2.8 [eV].

Having been accelerated in the manner described above, the pulsedcluster ion beam A is converged as appropriate by the primary-ion lens15, and is incident on the sample 2. As a result, neutral particles,electrons, and secondary ions are emitted from the surface of the sample2. While an incident angle at which the cluster ion beam A is incidenton the surface of the sample 2 is less than 90 degrees (i.e., parallelto the surface of the sample 2) and no less than 0 degree; when thecluster ion beam A is incident obliquely relative to the surface of thesample 2, the cluster ion beam A can be prevented from colliding with anextraction electrode 41.

As illustrated in FIG. 1B, the charged particle optical system 4, whichis disposed so as to face the sample 2, includes an extraction electrode41, a first projection electrode 42, a second projection electrode 43,and a flight-tube electrode 45. The second projection electrode 43 isdisposed on a side of the first projection electrode 42 that is oppositeto where the sample 2 is disposed. Each of the extraction electrode 41,the first projection electrode 42, the second projection electrode 43,and the flight-tube electrode 45 includes an opening for allowingcharged particles to pass therethrough. During operation, an appropriatevoltage is applied to each of the extraction electrode 41, the firstprojection electrode 42, the second projection electrode 43, and theflight-tube electrode 45. In particular, the potential of the extractionelectrode 41 relative to the potential of the sample 2 is referred to asan extraction voltage, and an electric field generated therebetween isreferred to as an extraction electric field.

Any of the extraction electrode 41, the first projection electrode 42,and the second projection electrode 43 may be hollow and conical with anopening formed at the vertex thereof (hereinafter, referred to as aconical electrode 411) (FIG. 6A), may be cylindrical, or may be a planaraperture electrode 412 having an opening formed therein as illustratedin FIG. 6B. In the present exemplary embodiment, a conical electrode isused as each of the first and second projection electrodes 42 and 43, asan example. In particular, if a conical electrode is used as theextraction electrode 41, a portion of the extraction electric field thatis spaced apart from the opening in the conical electrode is weaker thananother portion of the extraction electric field that is in the vicinityof the opening, which brings about an effect of suppressing deflectionof the primary ions through the extraction electric field or variationof the beam shape.

In the present exemplary embodiment, a cylindrical flight-tube electrode451, which is a cylindrical electrode, is used as the flight-tubeelectrode 45 (FIG. 6C); however, a multi-cylinder flight-tube electrode452, which is formed by a plurality of cylindrical electrodes, mayinstead be used as the flight-tube electrode 45 (FIG. 6D).Alternatively, an aperture flight-tube electrode 453, which is formed bystacking a plurality of aperture electrodes, may be used as theflight-tube electrode 45 (FIG. 6E).

As another alternative, an aperture-equipped flight-tube electrode 46,which is formed by connecting the second projection electrode 43 and theflight-tube electrode 45, may be used in place of the second projectionelectrode 43 and the flight-tube electrode 45 (FIG. 1E). Theaperture-equipped flight-tube electrode 46 includes, at an end thereof,a planar portion having an opening formed therein. The aperture-equippedflight-tube electrode 46 exhibits a function that is substantiallyequivalent to a function obtained by applying equal voltages to thesecond projection electrode 43 and the flight-tube electrode 45.Although an aperture electrode is used as each of the first and secondprojection electrodes 42 and 43 in the modification illustrated in FIG.1E, conical electrodes may instead be used.

The area of a region on the sample 2 that is to be irradiated with thecluster ion beam A (irradiation spot size) may be substantially equal tothe size of the opening formed in the extraction electrode 41, or may begreater than the size of the opening. In the latter case, it isadvantageous that the area in which the measurement can be carried outcan be increased by decreasing magnification without changing thepositional relationship between the sample 2 and the charged particleoptical system 4.

The secondary ions emitted from the sample 2 are accelerated through theextraction electric field, and enter the charged particle optical system4. The secondary ions are then converged through electric fields formedby the extraction electrode 41, the first projection electrode 42, andthe second projection electrode 43. Thin solid lines B indicate thetrajectories of the secondary ions that have been emitted from thesample 2 and reach the ion detector 5 via the charged particle opticalsystem 4. The secondary ions that have passed through the secondprojection electrode 43 enter the flight-tube electrode 45 having anequipotential space thereinside, and move uniformly therethrough. Thesecondary ions B that have passed through the flight-tube electrode 45are detected by the position- and time-sensitive ion detector 5 disposedat an exit of the flight-tube electrode 45. The ion detector 5transmits, to a signal processing system, the signal intensities of thesecondary ions along with the time at which the secondary ions aredetected.

The time from when the secondary ions are emitted from the sample 2 towhen the secondary ions are detected after having passed through thecharged particle optical system 4 (time of flight) can be obtained as adifference between the time at which the secondary ions are generatedand the time at which the secondary ions are detected by the iondetector 5. In the present exemplary embodiment, the time at which thecluster ion beam A is incident on the sample 2 can be regarded as thetime as which the secondary ions are emitted, and the time of flight ofthe secondary ions can thus be measured. As a result, the mass-to-chargeratio (m/z) of the secondary ions can be measured, or in other words,mass spectrometry of the secondary ions can be carried out.

In particular, if the flight-tube electrode 45 is sufficiently long inrelation to the lengths of the other electrodes, the mass-to-chargeratio of the secondary ions can be obtained approximately as follows.That is, the length of the flight-tube electrode 45 is substituted for Lin Equation 1; in a similar manner, the time from when the cluster ionbeam A is incident on the sample 2 to when the secondary ions aredetected by the ion detector 5 is substituted for t.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \mspace{500mu}} & \; \\{\frac{m}{z} = {2\mspace{14mu} {{eV}\left( \frac{t}{L} \right)}^{2}}} & \left( {{EQUATION}\mspace{14mu} 1} \right)\end{matrix}$

In Equation 1, m represents the mass of an ion; z represents the valenceof the ion; V represents the acceleration voltage; and e represents theelementary charge.

When the secondary ions reach the ion detector 5, an image of thesecondary ions on the sample 2 is formed on the surface of the iondetector 5 through the convergence effect of the charged particleoptical system 4. In other words, a point on the sample 2 is in acorrespondence relationship with another point on the surface of the iondetector 5, and thus each point on the sample 2 can be subjected to massspectrometry. Consequently, imaging mass spectrometry can be carriedout. An optical system having such a relationship is referred to as aso-called projection optical system, and a projection optical system isadvantageous in that the distribution of substances in a sample can beobtained without scanning the sample with a primary ion beam.

In the present exemplary embodiment, the charged particle optical system4 has two operation modes, namely, the high-magnification mode and thelow-magnification mode. As an example, in the high-magnification mode,voltages V1H, V2H, V3H, and VFH are applied, respectively, to theextraction electrode 41, the first projection electrode 42, the secondprojection electrode 43, and the flight-tube electrode 45 (FIG. 1C).Meanwhile, in the low-magnification mode, voltages V1L, V2L, V3L, andVFL are applied, respectively, to the extraction electrode 41, the firstprojection electrode 42, the second projection electrode 43, and theflight-tube electrode 45 (FIG. 1D). The positions of the extractionelectrode 41, the first projection electrode 42, the second projectionelectrode 43, the flight-tube electrode 45, and the ion detector 5 areindicated, respectively, by x0, x1, x2, xF, and xD, with the sample 2serving as a reference.

A voltage V0 is applied to the sample 2. The value of V0 may be 0 [V] ormay be in a range from positive/negative several [V] topositive/negative several tens of [V]. In addition, a voltage Vd havingan appropriate potential difference relative to V0 is applied to thesecondary ion detection surface of the ion detector 5.

As schematically illustrated in FIG. 1B, a principal plane PP, whichindicates the convergence effect of the projection-type charged particleoptical system 4 on the secondary ions, is formed in a travel path ofthe secondary ions and along a plane that is located between the firstprojection electrode 42 and the second projection electrode 43 and thatis orthogonal to the center axes of the openings formed in the firstprojection electrode 42 and the second projection electrode 43.

As illustrated in FIG. 1C, in the high-magnification mode, the potentialdifference between V1H and V2H is set to be greater than the potentialdifference between V2H and V3H. Thus, a first principal plane PPH isformed between the first projection electrode 42 and the secondprojection electrode 43.

In the meantime, as illustrated in FIG. 1D, in the low-magnificationmode, the potential difference between V1L and V2L is set to be smallerthan the potential difference between V2L and V3L. Thus, a secondprincipal plane PPL is formed between the second projection electrode 43and the flight-tube electrode 45.

In other words, with the projection-type charged particle optical systemaccording to the present exemplary embodiment, a principal plane can beformed variably at at least two positions in the travel path of chargedparticles by setting potentials to be applied to the respectiveelectrodes in each mode.

Ion-optical simulations have been conducted in order to illustrate theabove-described relationship in detail (FIGS. 3A, 3B, 3C, 3D, 4A, 4B,and 4C). It is to be noted that calculations are made under theassumption that the ion has a positive charge.

The extraction electrode 41 is disposed at a position that is spacedapart from the surface of the sample 2 by 2 [mm]. The extractionelectrode 41 is a conical electrode having a vertical angle of 70degrees and having an opening formed at the vertex. The opening is 2[mm] in diameter, and the outer diameter of the extraction electrode 41is 10 [mm].

The first projection electrode 42 is disposed at a position that isspaced apart from the extraction electrode 41 by 2 [mm]; and the secondprojection electrode 43 is disposed at a position that is spaced apartfrom the first projection electrode 42 by 2 [mm]. The shapes of thefirst projection electrode 42 and the second projection electrode 43 areidentical to the shape of the extraction electrode 41. It is to be notedthat each of the aforementioned distances between the adjacentelectrodes is based on a distance between the corresponding openings.

Furthermore, the flight-tube electrode 45 is disposed at a position thatis spaced apart from the second projection electrode 43 by 2 [mm], andthe flight-tube electrode 45 is a cylindrical electrode having an innerdiameter of 10 [mm] and a length of 50 [mm]. It is to be noted that theextraction electrode 41, the first projection electrode 42, the secondprojection electrode 43, and the flight-tube electrode 45 are disposedcoaxially.

In the high-magnification mode, as an example, V0 of 0 [V], V1H of −4000[V], and V2H of −500 [V] are applied. In addition, V3H of −1000 [V], VFHof −1000 [V], and Vd of −1000 [V] are applied. In the present exemplaryembodiment, a potential difference between the extraction electrode 41and the first projection electrode 42 is smaller than a potentialdifference between the sample and the extraction electrode 41. However,a potential difference between the extraction electrode 41 and the firstprojection electrode 42 may be larger than a potential differencebetween the sample and the extraction electrode 41. In thehigh-magnification mode, the potential difference between the extractionelectrode 41 and the first projection electrode 42 is larger than apotential difference between the first projection electrode 42 andsecond projection electrode 43. As indicated by thin solid lines B inFIG. 3A, secondary ions emitted from the sample 2 travel through thecharged particle optical system 4, and are then detected by the iondetector 5. FIG. 3B is an enlarged view of the vicinity of the sample 2.The secondary ions are emitted from three points on the sample 2 withemission-angle distribution. It can be seen that the secondary ionsemitted from the aforementioned three points are converged at respectivethree points on the ion detector 5, or in other words, an image of thesecondary ions is formed (FIG. 3A).

As illustrated in FIG. 3C, when looking at the trajectory of a secondaryion that has been emitted from the sample 2 in a direction parallel tothe optical axis of the charged particle optical system 4 and reachesthe ion detector 5, it is possible to draw an asymptote of the statedtrajectory extending from the sample 2 and another asymptotic of thestated trajectory extending from the ion detector 5. A point at which aperpendicular that extends from an intersection of the two asymptotestoward the center axis of the charged particle optical system 4intersects with the center axis is referred to as a principal point. Aplane that contains the principal point and that is orthogonal to thecenter axis is the principal plane. The position of the principal planecan be moved along the center axis by adjusting the voltages to beapplied to the respective electrodes. It is to be noted that FIG. 3Cillustrates the principal plane PPH in the high-magnification mode.

A projection magnification M of the charged particle optical system 4can be obtained through Equation 2 on the basis of a distance f1 betweenthe sample 2 and the principal plane and a distance f2 between theprincipal plane and the surface of the ion detector 5 (FIG. 3A).

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack \mspace{500mu}} & \; \\{M = \frac{f\; 2}{f\; 1}} & \left( {{EQUATION}\mspace{14mu} 2} \right)\end{matrix}$

In FIG. 3A, f1 is 6 [mm]; f2 is 57 [mm]; and the projectionmagnification MH in the high-magnification mode is thus 9.5×.

It is to be noted that the distances between given electrodes and thedistance between the sample 2 and the charged particle optical system 4are not limited to the aforementioned values, and may be modified.

Although a configuration may be such that the positional relationshipamong the electrodes is fixed, the charged particle optical system 4 maybe configured such that the distances from the sample 2 to therespective electrodes can be varied.

For example, the distance between the surface of the sample 2 and theextraction electrode 41 may be reduced to 1 [mm] from 2 [mm], and theremaining distances may be set the same as those described above. Insuch a case, the distance between the principal plane and the sample 2can be reduced; thus, f1 can be reduced, and f2 can be increased.Consequently, the projection magnification can be increased. Meanwhile,the projection magnification can be reduced by increasing the distancebetween the surface of the sample 2 and the extraction electrode 41.

In the meantime, in the low-magnification mode, V0 of 0 [V], V1L of−4000 [V], and V2L of −4800 [V] are applied. In addition, V3L of −1000[V], VFL of −1000 [V], and Vd of −1000 [V] are applied. As a result, thepotential difference between the extraction electrode 41 and the firstprojection electrode 42 is smaller than the potential difference betweenthe first projection electrode 42 and second projection electrode 43. Asdescribed above, a principal plane PPL is formed between the secondprojection electrode 43 and the flight-tube electrode 45 (FIG. 4A).Here, f1 is 9 [mm]; f2 is 54 [mm]; and the projection magnification MLin the low-magnification mode is thus 6×.

In both the high-magnification mode and the low-magnification mode, thepotential of the flight-tube electrode 45 and the potential of thesurface (detection surface) of the ion detector 5 on which an ion isincident may be equal to each other or may be different from each other.In particular, when an ion has a positive charge, the potential of thedetection surface may be lower than the potential of the flight-tubeelectrode 45. In such a case, the ion is accelerated through theelectric field generated between the flight-tube electrode 45 and thedetection surface, and the kinetic energy of the ion held when the ionis incident on the detection surface increases. This is advantageous inthat the sensitivity of the ion detector 5 improves. Meanwhile, thesensitivity improves in a similar manner if the potential of thedetection surface is higher than the potential of the flight-tubeelectrode 45 when an ion has a negative charge.

When looking at the electric field inside the flight-tube electrode 45,spacing between equipotential lines C inside the flight-tube electrode45 is large in both the high-magnification mode (FIG. 3D) and thelow-magnification mode (FIG. 4C). Thus, it can be seen that the spaceinside the flight-tube electrode 45 is substantially equipotential inboth the high-magnification mode and the low-magnification mode.Therefore, even if the projection magnification M is modified byadjusting the voltages to be applied to the respective electrodes, avariation in the time of flight t of secondary ions having the samemass-to-charge ratio is reduced. As a result, an influence on themeasured value of the mass-to-charge ratio is suppressed.

When V1L and V1H are set to the same potential as in the presentexemplary embodiment, the extraction electric field present between thesample 2 and the extraction electrode 41 substantially does not change.Thus, a variation in the efficiency with which the secondary ions passthrough the extraction electrode 41 can advantageously be suppressed. Ina similar manner, a variation in the trajectory of the primary ions dueto the extraction electric field can advantageously be suppressed aswell.

Although VFL and VFH may be set to difference voltages, if VFL and VFHare set to the same potential as in the present exemplary embodiment,the time for which an ion travels through the flight-tube electrode 45substantially does not change. Therefore, advantageously, a variation inthe time of flight t can be substantially ignored. In particular, if theflight-tube electrode 45 is sufficiently long relative to the otherelectrodes, the time of flight t substantially does not vary even whenthe projection magnification M is changed. Thus, the accuracy inmeasuring the mass-to-charge ratio advantageously increases.

Meanwhile, the accuracy in measuring the mass-to-charge ratio isexpressed through Equation 3; thus, when the time of flight t changes,mass resolution Δm/m is also affected. Here, Δt represents the accuracyin measuring the time of flight t. As described above, when a variationin the time of flight t decreases, a variation in the mass resolutioncan also be suppressed.

$\begin{matrix}{\left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack \mspace{500mu}} & \; \\{\frac{\Delta \; m}{m} = {2\frac{\Delta \; t}{t}}} & \left( {{EQUATION}\mspace{14mu} 3} \right)\end{matrix}$

As described above, the projection-type charged particle optical systemthat is capable of measuring the mass-to-charge ratio with high accuracyeven when the projection magnification is changed can be provided.

Here, an ion-optical simulation of a conventional projection-typecharged particle optical system 400 (FIG. 2A) has been conducted forcomparison, and the projection-type charged particle optical system 400consists of the extraction electrode 41, the first projection electrode42, and the flight-tube electrode 45. Results are illustrated in FIGS.2B through 2E. The distance between the first projection electrode 42and the flight-tube electrode 45 is 2 [mm]. Aside from a feature thatthe second projection electrode 43 is not provided, the shapes and thesizes of the respective electrodes are the same as those in thesimulation indicated in FIGS. 3A through 3D.

A result obtained when the projection-type charged particle opticalsystem 400 is operated in the high-magnification mode is illustrated inFIG. 2B. Concerning the voltage settings, V0 of 0 [V], V1H of −4000 [V],and V2H of −520 [V] are applied. In addition, VFH of −1000 [V] and Vd of−1000 [V] are applied. The principal plane PPH is formed between thefirst projection electrode 42 and the flight-tube electrode 45, and animage of the secondary ions is formed on the ion detector 5.

In the meantime, in order to reduce the magnification, the principalplane PPL is moved toward the ion detector 5 in FIG. 2C. In order toreduce the projection magnification while using the electrodesillustrated in FIG. 2B, the potential difference between the firstprojection electrode 42 and the flight-tube electrode 45 needs to be setgreater than the potential difference between the extraction electrode41 and the first projection electrode 42. Therefore, V0 of 0 [V], V1L of−4000 [V], V2L of −12500 [V], VFL of −1000 [V], and Vd of −1000 [V] areapplied. A comparison of the potential distributions at an entry of theflight-tube electrode 45 reveals that the potential gradient is steeperin the low-magnification mode (FIG. 2E) than in the high-magnificationmode (FIG. 2D). Here, smaller spacing of the equipotential lines C isassociated with a steeper potential gradient.

Consequently, ions are accelerated or decelerated through the electricfield inside the flight-tube electrode 45; thus, the time of flight ofthe ions varies between the two modes. In other words, in theconventional technique, changing the magnification leads to a variationin the time of flight of the secondary ions inside the flight-tubeelectrode 45; thus, the conventional technique has such shortcomingsthat the measured value of the mass-to-charge ratio or the resolutionvaries.

As described thus far, the conventional projection-type charged particleoptical system has shortcomings that changing the projectionmagnification lead to a decrease in the accuracy in measuring themass-to-charge ratio. On the other hand, according to the presentinvention, a variation in the time of flight t of the secondary ions isreduced, which makes it possible to measure the mass-to-charge ratiowith high accuracy even when the projection magnification is changed.

Although a cluster ion is illustrated as an example of the primary ionin the present exemplary embodiment, the present invention can also beapplied to other charged substances, such as a molecular ion, afullerene ion, and a charged liquid-droplet. In addition, although thecharged particle optical system is used for mass spectrometry of ions inthe present exemplary embodiment, the charged particle optical systemcan also be used for mass spectrometry of other charged particles.

Second Exemplary Embodiment

A projection-type imaging mass spectrometry apparatus according to thepresent exemplary embodiment is similar to the projection-type imagingmass spectrometry apparatus according to the first exemplary embodimentexcept for the configuration of a projection-type charged particleoptical system.

Unlike the projection-type charged particle optical system 4 accordingto the first exemplary embodiment, a projection-type charged particleoptical system 4 according to the present exemplary embodiment furtherincludes a third projection electrode 44, which is disposed between thesecond projection electrode 43 and the flight-tube electrode 45. Inaddition, voltages to be applied to each of the electrodes differ fromthe voltages in the first exemplary embodiment. The distance between thesecond projection electrode 43 and the third projection electrode 44 is2 [mm], and the distance between the third projection electrode 44 andthe flight-tube electrode 45 is 2 [mm]. FIGS. 5A through 5F illustrateresults of an ion-optical simulation of a projection imaging massspectrometer according to the present exemplary embodiment.

In the high-magnification mode (FIG. 5A), as an example, V0 of 0 [V],V1H of −4000 [V], and V2H of −570 [V] are applied to the respectiveelectrodes. In addition, V3H of −1000 [V], VFH of −1000 [V], and Vd of−1000 [V] are applied. Furthermore, a voltage V4H of −1000 [V] isapplied to the third projection electrode 44. Thin solid lines B in FIG.5A indicate the trajectories of the secondary ions that have beenemitted from the sample 2 and reach the ion detector 5 through thecharged particle optical system 4. As in the first exemplary embodiment,it can be seen that the secondary ions are emitted from three points onthe sample 2 with emission-angle distribution and converge at respectivethree points on the ion detector 5. In FIG. 5A, the principal plane PPHis formed between the first projection electrode 42 and the secondprojection electrode 43 in the high-magnification mode. Here, f1 is 6[mm]; f2 is 57 [mm]; and the projection magnification MH is thus 9.5×.

In the meantime, in the low-magnification mode, as an example, V0 of 0[V], V1L of −4000 [V], V2L of −4000 [V], and V3L of −3900 [V] areapplied. In addition, VFL of −1000 [V] and Vd of −1000 [V] are applied.Furthermore, a voltage V4L of −1000 [V] is applied to the thirdprojection electrode 44. The principal plane PPL is formed between thethird projection electrode 44 and the flight-tube electrode 45 in thelow-magnification mode. Here, f1 is 11 [mm]; f2 is 52 [mm]; and theprojection magnification ML in the low-magnification mode is thus 4.7×(FIG. 5B).

Furthermore, in the present exemplary embodiment, another principalplane PPM is formed between the second projection electrode 43 and thethird projection electrode 44, and the projection imaging massspectrometer is thus provided with a medium-magnification mode in whichan image of secondary ions is formed with a medium magnification. In themedium-magnification mode, a voltage of 0 [V] is applied to the sample2; a voltage of −4000 [V] is applied to the extraction electrode 41; anda voltage of −4750 [V] is applied to the first projection electrode 42.A voltage of −1000 [V] is applied to each of the second projectionelectrode 43, the third projection electrode 44, the flight-tubeelectrode 45 and the ion detector 5. Here, f1 is 9 [mm]; f2 is 54 [mm];and the projection magnification in the medium-magnification mode isthus 6× (FIG. 5C).

In any of the high-magnification mode (FIG. 5D), the low-magnificationmode (FIG. 5E), and the medium-magnification mode (FIG. 5F), spacing ofthe equipotential lines C inside the flight-tube electrode 45 is large,and this reveals that the space inside the flight-tube electrode 45 is asubstantially equipotential space. Therefore, even if the thirdprojection electrode 44 is added and the projection magnification isthus modified in three levels, a variation in the time of flight t ofthe secondary ions having the same mass-to-charge ratio is reduced. As aresult, the mass-to-charge ratio can be measured with high accuracy.

Although the projection-type charged particle optical system 4 accordingto the present exemplary embodiment includes three projectionelectrodes, the projection-type charged particle optical system 4 mayinclude four or more projection electrodes.

In other words, with the charged particle optical system according tothe present exemplary embodiment, a principal plane can be formed at atleast two positions along the trajectory of the charged particles, andthree or more principal planes can be formed.

Third Exemplary Embodiment

A projection-type imaging mass spectrometry apparatus (FIG. 6F)according to the present exemplary embodiment irradiates the sample 2with a laser beam, which serves as the primary beam. The third exemplaryembodiment is similar to the first exemplary embodiment except in thatat least a portion of the sample 2 is ionized and that the generatedions are subjected to mass spectrometry.

A laser beam source 16 may be an ultraviolet laser or a visible laser. Alaser beam D passes through an optical window 17, and is incident on thesample 2 inside the vacuum vessel 6; thus, ions are emitted from thesurface of the sample 2. A matrix agent may be applied to the sample 2.

An image of the ions emitted from the sample 2 is formed on the iondetector 5 by the charged particle optical system 4, as in the firstexemplary embodiment. In the present exemplary embodiment, the laserbeam D is a pulsed laser beam, and the ions are thus emitted in pulses.Therefore, an interval between the time at which the sample 2 isirradiated with the laser beam D and the time in which the ions aredetected by the ion detector 5 corresponds to the time of flight.

In this manner, an image of the ions emitted from the sample 2 that isirradiated with the laser beam D is formed on the ion detector 5, andthe time of flight of the ions is measured. Thus, imaging massspectrometry can be carried out.

With the projection-type imaging mass spectrometry apparatus accordingto the present exemplary embodiment, the mass-to-charge ratio can bemeasured with high accuracy even when the projection magnification ischanged. In addition, a sample to which a matrix agent has been appliedis irradiated with a laser beam, and thus macromolecules, such as abiomolecule, can be detected with high sensitivity.

Fourth Exemplary Embodiment

A projection-type imaging mass spectrometry apparatus according to thepresent exemplary embodiment is similar to the projection-type imagingmass spectrometry apparatus according to the second exemplary embodimentexcept for the configuration of a projection-type charged particleoptical system.

Unlike the projection-type charged particle optical system 4 accordingto the second exemplary embodiment, as illustrated in FIG. 6G, aprojection-type charged particle optical system 4 according to thepresent exemplary embodiment includes a fourth projection electrode 47,which is disposed between the third projection electrode 44 and theflight-tube electrode 45. In addition, voltages to be applied to each ofthe electrodes differ from the voltages in the second exemplaryembodiment. The distance between the third projection electrode 44 andthe fourth projection electrode 47 is 2 [mm], and the distance betweenthe fourth projection electrode 47 and the flight-tube electrode 45 is 2[mm]. FIGS. 7A through 7D illustrate results of an ion-opticalsimulation of a projection imaging mass spectrometer according to thepresent exemplary embodiment.

In the high-magnification mode (FIG. 7A), as an example, V0 of 0 [V],V1H of −4000 [V], and V2H of −580 [V] are applied to the respectiveelectrodes. In addition, V3H of −1000 [V], V4H of −900 [V], VFH of −1000[V], and Vd of −1000 [V] are applied. Furthermore, a voltage V5H of−1000 [V] is applied to the fourth projection electrode 47. Thin solidlines B in FIG. 7A indicate the trajectories of the secondary ionsemitted from the sample 2. As in the second exemplary embodiment, it canbe seen that the secondary ions are emitted from three points on thesample 2 with emission-angle distribution and converge at respectivethree points on the ion detector 5. In FIG. 7A, the principal plane PPHis formed between the first projection electrode 42 and the secondprojection electrode 43 in the high-magnification mode. Here, f1 is 6[mm]; f2 is 63 [mm]; and the projection magnification MH is thus 10.5×.

In the meantime, in the low-magnification mode, as an example, V0 of 0[V], V1L of −4000 [V], and V2L of −2000 [V] are applied. In addition,V3L of −1000 [V], V4L of −550 [V], VFL of −1000 [V], and Vd of −1000 [V]are applied. Furthermore, a voltage V5L of −1000 [V] is applied to thefourth projection electrode 47. The principal plane PPL is formedbetween the fourth projection electrode 47 and the flight-tube electrode45 in the low-magnification mode. Here, f1 is 15 [mm]; f2 is 54 [mm];and the projection magnification ML in the low-magnification mode isthus 3.6× (FIG. 7B).

One of the features of the present exemplary embodiment is that V3L andV3H do not change even when the high-magnification mode (FIG. 7C) andthe low-magnification (FIG. 7D) mode are switched, as described above.Therefore, even when the voltage applied to the first projectionelectrode 42 is changed, it is possible to reduce an electric fieldformed by the first projection electrode 42 in a space on a side of thesecond projection electrode 43 where the third projection electrode 44is disposed. Meanwhile, even when the voltage applied to the thirdprojection electrode 44 is changed, it is possible to reduce an electricfield formed by the third projection electrode 44 in a space on a sideof the second projection electrode 43 where the first projectionelectrode 42 is disposed. Consequently, electric fields for convergingthe ions can be formed with high accuracy, and the aberration of theelectrostatic lens can be reduced. Accordingly, the spatial resolutioncan advantageously be increased.

The projection-type charged particle optical system 4 also has themedium-magnification mode, as in the second exemplary embodiment, and asecond medium-magnification mode can be generated in which a principalplane is formed at a position different from any of the positions wherea principal plane is formed in the above-described three modes, byadjusting voltages to be applied to the respective electrodes. In anexemplary case, a voltage that is different from either of V3L and V3His applied to the second projection electrode 43. In this manner, aprincipal plane can be formed at at least two positions along thetrajectory of the charged particles, and three or more principal planescan be formed.

Although the projection-type charged particle optical system 4 accordingto the present exemplary embodiment includes four projection electrodes,the projection-type charged particle optical system 4 may include fiveor more projection electrodes.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. A projection-type charged particle optical system, comprising: afirst electrode disposed so as to face a sample, the first electrodehaving an opening formed therein for allowing a charged particle to passtherethrough; a second electrode disposed on a side of the firstelectrode, the side being opposite to where the sample is disposed, thesecond electrode having an opening formed therein for allowing thecharged particle to pass therethrough; and a flight-tube electrodedisposed such that the charged particle that has been emitted from thesample and has passed through the first and second electrodes enters theflight-tube electrode, the flight-tube electrode being configured toform a substantially equipotential space thereinside, wherein theprojection-type charged particle optical system has an operation mode inwhich a potential applied to the first electrode is higher than apotential of the flight-tube electrode and a potential applied to thesecond electrode is lower than the potential of the light-tubeelectrode.
 2. The projection-type charged particle optical systemaccording to claim 1, further comprising a third electrode disposedbetween the second electrode and the flight-tube electrode, the thirdelectrode having an opening formed therein for allowing the chargedparticle to pass therethrough, wherein a projection magnification ischanged in such a manner that without a change in potentials of thefirst electrode and the flight-tube electrode, potentials of the secondelectrode and the third electrode are changed.
 3. The projection-typecharged particle optical system according to claim 1, further comprisinga third electrode disposed between the second electrode and theflight-tube electrode, the third electrode having an opening formedtherein for allowing the charged particle to pass therethrough, whereinthe projection-type charged particle optical system has a firstoperation mode and a second operation mode, the first operation modebeing a mode in which the potential applied to the first electrode ishigher than the potential of the flight-tube electrode, the potentialapplied to the second electrode is lower than the potential of theflight-tube electrode, and a potential applied to the third electrode isequal to the potential of the flight-tube electrode, the secondoperation mode being a mode in which the potential applied to the firstelectrode is higher than the potential of the flight-tube electrode, thepotential applied to the second electrode is higher than the potentialof the flight-tube electrode, and the potential applied to the thirdelectrode is lower than the potential of the flight-tube electrode. 4.The projection-type charged particle optical system according to claim1, wherein at least one of the first and second electrodes is a hollowcone, the opening being located at a vertex of the hollow cone.
 5. Theprojection-type charged particle optical system according to claim 1,wherein a potential difference between the first and second electrodesis smaller than a potential difference between the sample and the firstelectrode.
 6. The projection-type charged particle optical systemaccording to claim 1, further comprising a third electrode disposedbetween the second electrode and the flight-tube electrode, the thirdelectrode having an opening formed therein for allowing the chargedparticle to pass therethrough.
 7. The projection-type charged particleoptical system according to claim 6, further comprising a fourthelectrode disposed between the third electrode and the flight-tubeelectrode, the fourth electrode having an opening formed therein forallowing the charged particle to pass therethrough.
 8. Theprojection-type charged particle optical system according to claim 7,further comprising a fifth electrode disposed between the fourthelectrode and the flight-tube electrode, the fifth electrode having anopening formed therein for allowing the charged particle to passtherethrough.
 9. The projection-type charged particle optical systemaccording to claim 8, wherein a projection magnification is changed insuch a manner that without a change in potentials of the first electrodeand the flight-tube electrode, at least any of potentials of the secondto the fifth electrodes is changed.
 10. The projection-type chargedparticle optical system according to claim 1, wherein the flight-tubeelectrode includes a planar member provided at an end thereof, theplanar member having an opening formed therein for allowing the chargedparticle to pass therethrough.
 11. A time-of-flight mass spectrometer,comprising: the projection-type charged particle optical systemaccording to claim 1; and a position- and time-sensitive detectorconfigured to detect the charged particle that has passed through theflight-tube electrode, wherein an image of the charged particle isformed on a surface of the position- and time-sensitive detector, and atime at which the charged particle has been detected is recorded in theposition- and time-sensitive detector.
 12. The time-of-flight massspectrometer according to claim 11, wherein a potential difference isgenerated between a detection surface of the position- andtime-sensitive detector on which the charged particle is incident andthe flight-tube electrode such that kinetic energy of the chargedparticle increases.
 13. A time-of-flight mass spectrometry apparatus,comprising: the time-of-flight mass spectrometer according to claim 11;and a pulsed charged particle source configured to generate chargedparticles from a surface of the sample in pulses.
 14. The time-of-flightmass spectrometry apparatus according to claim 13, wherein a size of aregion on the surface of the sample from which the charged particles arecaused to be emitted by the pulsed charged particle source is equal toor greater than a size of the opening formed in the first electrode. 15.A method for projecting a charged particle with a projection-typecharged particle optical system that includes a first electrode disposedso as to face a sample and having an opening formed therein for allowingthe charged particle to pass therethrough, a second electrode disposedon a side of the first electrode that is opposite to where the sample isdisposed and having an opening formed therein for allowing the chargedparticle to pass therethrough, and a flight-tube electrode disposed suchthat the charged particle that has been emitted from the sample and haspassed through the first and second electrodes enters the flight-tubeelectrode and being configured to form a substantially equipotentialspace thereinside, the method comprising: causing, in theprojection-type charged particle optical system, the charged particle totravel in an operation mode in which a potential applied to the firstelectrode is higher than a potential of the flight-tube electrode and apotential applied to the second electrode is lower than the potential ofthe flight-tube electrode.
 16. The method for projecting a chargedparticle according to claim 15, wherein a third electrode is furtherdisposed between the second electrode and the flight-tube electrode, thethird electrode having an opening formed therein for allowing thecharged particle to pass therethrough, and wherein the method includes achanging step in which a projection magnification is changed in such amanner that without a change in potentials of the first electrode andthe flight-tube electrode, potentials of the second electrode and thethird electrode are changed.
 17. The method for projecting a chargedparticle according to claim 16, the method comprising: causing thecharged particle to travel in a first operation mode in which thepotential applied to the first electrode is higher than the potential ofthe flight-tube electrode, the potential applied to the second electrodeis lower than the potential of the flight-tube electrode, and apotential applied to the third electrode is equal to the potential ofthe flight-tube electrode, and causing the charged particle to travel ina second operation mode in which the potential applied to the firstelectrode is higher than the potential of the flight-tube electrode, thepotential applied to the second electrode is higher than the potentialof the flight-tube electrode, and the potential applied to the thirdelectrode is lower than the potential of the flight-tube electrode. 18.The method for projecting a charged particle according to claim 17,wherein a fourth electrode is further disposed between the thirdelectrode and the flight-tube electrode, the fourth electrode having anopening formed therein for allowing the charged particle to passtherethrough, and wherein a position at which a principal plane isformed is changed by changing of a potential of each of the first,second, third, and fourth electrodes.
 19. The method for projecting acharged particle according to claim 18, wherein a fifth electrode isfurther disposed between the fourth electrode and the flight-tubeelectrode, the fifth electrode having an opening formed therein forallowing the charged particle to pass therethrough, and wherein theposition at which the principal plane is formed is changed by changingof a potential of each of the first, second, third, fourth, and fifthelectrodes.